This invention relates to defect detection apparatus for visually inspecting and measuring the features of an object, and, more particularly, to such apparatus for inspecting a perforated object for etch defects and measuring the dimensions of such defects.
Despite efforts to insure uniformity, modern manufacturing processes continue to mass-produce products with varying characteristics. Inspection mechanisms are utilized to sort units of product according to the individual characteristics of each. In some applications, both objective and subjective quality standards are applied to judge a product's characteristics. Extremely fine gradations of quality can pose a difficult inspection challenge.
In some industries, teams of human inspectors are employed to inspect the output of mass-production manufacturing to determine the compliance of each unit of product with the applicable standards. Relative to automated manufacturing, the process of human inspection has proven to be slow, inefficient, and costly, thereby limiting overall manufacturing throughput. In some industrial settings human inspection is entirely infeasible because the demands for speed and accuracy simply exceed human capacity.
At present, human inspectors are still utilized by video display manufacturers to inspect video display components. A particularly time-consuming and exhausting task is the inspection of aperture grills. An example of an aperture grill 201 is illustrated in FIGS. 1A, 1B, 1C, and 1D. An aperture grill is used during the manufacture of video displays to mask the interior surface of a display screen for the application of phosphorus coatings. Also, aperture grills are physically incorporated into video display screens as a means for focusing an incident beam of electrons from an electron gun.
Typically, an aperture grill 201 is comprised of a thin sheet 202 of material, preferably metal, in which a number of openings 203 and 204 have been formed. As depicted in FIG. 1A, the aperture grill 201 includes a number of openings 203, preferably substantially rectangular. Long, thin and substantially rectangular openings are referred to as "slits". As illustrated, each of the slits 203 includes two respective "slit ends" 207 located at opposite ends of the slit. The long, thin portions of the sheet 202 that remain interspersed among the openings 203 are called "ribbons" 206, also referred to as "tapes". In the following discussion, the terms "slits" and "ribbons" should be understood to include "openings" and "tapes", respectively.
In a preferred embodiment, the width of each of slits 203 is between 140 and 260 microns and the width of each of ribbons 206 is between 500 and 1000 microns. These ranges are given merely as examples, as an aperture grill need not be so limited.
At each end of the aperture grill 201, and substantially parallel to the slits 203, is a breakaway tab 204. Each breakaway tab 204 preferably consists of an opening with two long straight sides and two curved ends 204A and 204B. An aperture grill may be perforated with a number of additional openings; however, description of such has been omitted in the interest of brevity.
FIG. 1B presents a magnified illustration of area B of FIG. 1A. FIG. 1B illustrates ribbons 206; slit ends 205, 205' and 205"; under-etch defects 208 and 208'; over-etch defects 209 and 209'; and bridge defect 208".
Etch defects 208, 208', 208", 209, and 209' result from process variations which occur during the etching of slits 203. As illustrated, under-etch defects 208, 208', and 208" result in an excess of material protruding from a ribbon 206 into a slit 203, interrupting the otherwise substantially straight edges of the slit. Similarly, over-etch defects 209 and 209" result in a localized void in a ribbon 206, increasing the width of the slit at that location, and interrupting the otherwise substantially straight edges of the slit. Bridge defect 208", an extreme under-etch defect, connects two of ribbons 206 with a "bridge" of material, as shown. Slit ends 205' and 205" illustrate irregular slit ends that can result from an imperfect etching process.
An etch defect of large enough size will render a particular aperture grill unsuitable for use. Specifically, an aperture grill containing a single etch defect having dimensions greater than 50 microns by 150 microns will cause an undesirable blemish on an operating video display into which the aperture grill is incorporated. As the detection of even smaller defects may be necessary in some applications, the present invention is not limited to the detection of defects of any particular dimensions. Further, it should be appreciated that any portion of an aperture grill may include one or more under-etch, over-etch, and bridge defects.
In FIG. 1C, the slits 210, 211, and 214 have approximately the same width (ignoring localized defects), whereas slit 212 is relatively wider and slit 213 is relatively thinner. Each of ribbons 206 has the same width (ribbon width 206').
A "pitch" may be defined as the distance between two corresponding edges of two adjacent pairs of edges 207 (e.g. the distance between the left-hand edges or the right-hand edges of two adjacent slits). Thus, there are at least two ways to define the pitches of an aperture grill. For example, pitch 212' is the distance between the right-hand edge of slit 211 and the right-hand edge of slit 212, and thus spans slit 212. Pitch 212" is the distance between the left-hand edge of slit 212 and the left-hand edge of slit 213.
As illustrated in FIG. 1C, it is apparent that the pitch (212', 212") spanning slit 212 is greater than the pitch (214', 214") spanning slit 214. In contrast, the pitch spanning slit 213 (213', 213") is less than the pitch spanning slit 214 (214', 214"). A pitch defect is defined as a pitch which varies substantially from the average of the pitches of an aperture grill. For a typical video display, pitch defects in excess of three percent of the average pitch of the aperture grill are noticeable to the human eye, and render the aperture grill unsuitable for use. Pitches 212' (212") and 213' (213") illustrate two types of pitch defects.
Another type of defect that occurs in aperture grills is a width defect. Examples of four width defects are provided in FIG. 1D. FIG. 1D presents a magnified illustration of area B of FIG. 1A. FIG. 1D illustrates ribbons 206, 206a, and 206b; ribbon widths 206', 206a', and 206b'; slits 215-217; and slit widths 215'-217'. As depicted, each of ribbons 206 is of the same width 206', whereas ribbon 206a has a greater width 206a' and ribbon 206b has a lesser width 206b'. Each of slits 215 is of the same width 215', whereas slit 216 has a lesser width 216' and slit 217 has a greater width 217'.
If it is assumed that width 206' is a typical ribbon width and width 215' is a typical slit width, then ribbon 206a demonstrates a first type of width defect as it is wider than ribbon 206, i.e. 206a' is greater than 206'. Ribbon 206b demonstrates a second type of width defect as it is thinner than ribbon 206, i.e. 206b' is less than 206'. Slit 216 demonstrates a third type of width defect as it is thinner than slit 215, i.e. 216' is less than 215'. Slit 217 demonstrates a fourth type of width defect as it is wider than slit 215, i.e. 217' is greater than 215'.
It should be appreciated that the concepts of pitch, pitch defect, and width defect may be defined in numerous ways and that the present invention is not limited to the above-described definitions. For example, pitch might be calculated as the distance between the centers of two adjacent slits or between the centers of two adjacent ribbons. A pitch defect might be defined as a pitch which falls outside a predetermined range of pitches.
Although automatic visual inspection systems have been created to meet the inspection demands of a variety of specific manufacturing applications, the inspection of aperture grills has yet to be automated. A visual inspection system 18 is illustrated in FIG. 2. As shown therein, such a system 18 includes a transport mechanism 22, a scanner 20, a processor 19, a sorter 24, a "good" bin 21, and a "bad" bin 26.
In the system 18 of FIG. 2, the transport mechanism 22 conveys a series of objects 23 through the scanning field of scanner 20 to the sorter 24. Scanner 20 acquires a visual image of each object 23 and sends the image to the processor 19 to be analyzed. The processor 19 analyzes the visual image corresponding to each of the objects 23 and classifies the particular object 23 as either "good" or "bad." Accordingly, the processor 19 controls the operation of sorter 24, causing it to route each object 23 to either the "good" bin 21 or the "bad" bin 26.
To date, aperture grills have been considered unsuitable for automatic visual inspection due to the limited resolution of available imaging technology and the considerable expense of hardware capable of processing vast amounts of image data at assembly-line speeds. Thus, an economically feasible automatic visual inspection system suitable for inspecting aperture grills for etch and pitch defects in a manufacturing environment is needed.